Conventionally, lubricating oil has been used in an internal combustion engine, a transmission or the like to facilitate the smooth operation thereof. In particular, a lubricating oil (engine oil) for an internal combustion engine is required to have a high level of performances because the internal combustion engine has been improved in performance, enhanced in output and used under severe working conditions. Therefore, conventional engine oils have been blended with various additives such as antiwear agents, metallic detergents, ashless dispersants, and anti-oxidants to meet such requisite performances. Since an engine oil contacts peroxides that are active species of oxidation degradation at piston-cylinder regions, it has been required to have excellent oxidation resistivity. Phenol- or amine-based anti-oxidants and zinc dialkyldithiophosphate (ZDTP) acting as a peroxide decomposer have, therefore, been used in an engine oil so as to enhance the oxidation stability thereof. The use of a molybdenum compound as a peroxide decomposer is also known. Whilst, it is also conventionally known to blend an hindered amine (for example, see Patent Literatures 1 and 2) and also known to blend a phenol-based anti-oxidant, or zinc dialkyldithiophosphate and hindered amine (for example, see Patent Literatures 3 to 5) in order to enhance the oxidation stability of a lubricating oil. However, it is not known to blend a hindered amine compound, and an organic molybdenum compound and further a phenol-based anti-oxidant at the same time so as to enhance the NOx resistance.
Iron-based materials and aluminum-based materials are mainly used as sliding materials, but aluminum-, tin-, copper- and/or lead-containing materials are sometimes used for the main bearing or con rod bearing of an internal combustion engine. Among these materials, lead-containing materials have excellent characteristics that they are less in fatigue phenomenon but have a drawback that they are large in corrosion wear. It is known that such corrosion is caused by accumulation of peroxide due to deterioration of the oil or oxidization directly by molecular oxygen in the air and also known that oxidation products such as quinone, diacetyl, nitrogen oxide, nitro compounds or the like facilitate corrosion in case of coexistence with acid.
Actual corrosion is complicated because it is governed by many factors, but for corrosion prevention, it is generally important to prevent oxidation of a lubricating oil, destroy oxidizing substances, prevent generation of anti-corrosive oxidation products, deactivate acidic substances and form anti-corrosive film on metal surfaces. As a more specific method for preventing corrosion, it is known to add to a lubricating oil an additive having functions as a peroxide decomposer and an anti-corrosive film forming agent such as zinc dithiophosphate or sulfide, an amine- or phenol-based chain terminating type anti-oxidant, an anti-corrosive film forming agent such as benzotriazole, and a detergent dispersant such as acid neutralizer, and most of these four types of components are used in combination.
In particular, for corrosion wear prevention of lead-containing sliding materials, sulfur-containing antiwear agents such as zinc dithiophosphate are extremely effective, and for example, a conventional engine oil blended with zinc dithiophosphate can perform a peroxide decomposition effect together with excellent lead corrosion wear prevention due to deactivation of lead surfaces (for example, see Patent Literature 6).
Whilst, a sulfur-containing compound such as zinc dithiophosphate is likely to cause sulfide corrosion with respect to sliding materials containing non-ferrous base metals other than lead (for example, copper, tin, silver and the like), and although corrosion inhibitors such as benzotriazole is effective in corrosion prevention of copper, it has been found that the sulfur-containing compound does not exhibit sufficient effects in preventing corrosion of lead. That is, it was impossible to prevent elution of copper and/or lead with prior art techniques.